1. Field of the Invention
The present invention relates to an automatic adjusting circuit for an analog filter used in an analog signal processing, and, more particularly, to an automatic adjusting circuit for automatically adjusting a center frequency of an analog filter with a high accuracy to be exactly equal to a reference frequency of an input signal.
2. Description of the Related Art
A conventional automatic adjusting circuit for an analog filter used in an analog phase synchronization loop circuit in a color sub-carrier signal system at 3.58 MHz of a color television receiver, for example, is arranged as shown in FIG. 1. In a semiconductor integrated circuit shown, reference numeral 41 designates an analog filter of a variable gain type, which receives an input signal Si of a reference frequency fs. The center frequency "fo" of the analog filter 41 is varied by controlling its gain (gm) in accordance with a bias input signal. The analog filter 41 is a low-pass filter, for example. A phase detector 42, which is made up of a multiplier, for example, phase detects an output signal of the analog filter 41 with reference to the input signal Si that is fed through a signal line 45, and outputs a component signal which depends on a phase difference. A smoothing capacitor 43, which is externally coupled with the integrated circuit, is for smoothing the output signal of the phase detector 42 to thus convert it into a DC signal. The phase detected output signal as the DC signal is fed back as a fine adjusting bias input signal to the analog filter 41 through a feedback loop 46. A variable resistor 44, which is externally coupled with the integrated circuit, is for applying a coarse adjusting bias input to the analog filter 41.
In the automatic adjusting circuit for the analog filter, in order to adjust the filter characteristic of the analog filter 41 s that the center frequency "fo" of the analog filter 41 is equal to the reference frequency "fs" of the input signal Si, first a coarse adjustment is made so that the center frequency "fo" of the analog filter 41 falls within a predetermined range with respect to the reference frequency "fs", by manually adjusting the external variable resistor 44. Then, the phase detector 42 automatically executes a fine adjustment of the analog filter 41 by its output signal.
However, the manual operation for the filter characteristic adjustment is attendant with many drawbacks. In particular, the adjusting work is very troublesome. Further, in case that the filter characteristic of the analog filter after adjusted is deviated from the correct one, it is often impossible to adjust such filter characteristic. Furthermore, since the variable resistor 44 is the part externally coupled with the integrated circuit, an operator will feel inconvenient in using the integrated circuit.
To make an automatic adjustment of the filter characteristic of the analog filter 41 over a broad frequency band, the frequency band must be broadened by lowering the selectivity Q of the filter. Therefore, it is impossible to make an exact fine adjustment of the filter characteristic.
In case where the analog filter 41 is a filter of a high selectivity Q, it is necessary to previously adjust and set the center frequency "fo" of the analog filter 41 in the vicinity of the reference frequency "fs" before an automatic fine adjustment. Also in this case, there still exist the drawback of the troublesome manual adjustment work and the drawback of frequently failing to take a countermeasure for the deviation of the adjusted filter characteristic.
As described above, the conventional automatic adjusting circuit for an analog filter has the following drawbacks. 1) Since the filter characteristic must be manually adjusted, the adjusting work is delicate and troublesome. 2) When, after adjusted, the filter characteristic is deviated from the correct one, there is a case that no measure can be taken for the deviated filter characteristic. 3) Since the variable resistor 44 is needed as the part externally coupled with the integrated circuit, a designer will feel inconvenient in circuit design using the integrated circuit. 4) To make an automatic adjustment of the analog filter over a broad frequency band, the frequency band must be broadened by lowering the selectivity Q of the filter. Accordingly, it is impossible to make a fine adjustment of the filter characteristic with a high accuracy.